1. Field of the Invention
The invention relates to an X-ray examination apparatus which includes an X-ray source, an X-ray detector and an X-ray filter which is arranged between the X-ray source and the X-ray detector and comprises a plurality of filter elements whose X-ray absorptivity can be adjusted by controlling a quantity of an X-ray absorbing liquid in individual filter elements.
2. Description of the Related Art
An X-ray examination apparatus of this kind is known from the international patent application WO 96/13040.
The X-ray examination apparatus forms an X-ray image of an object to be examined, for example a patient to be radiologically examined. The X-ray source irradiates the object by means of an X-ray beam and an X-ray image is formed on the X-ray detector due to local differences in X-ray absorption within the object. The X-ray filter ensures that the range of brightness values of the X-ray image remains limited. The X-ray filter is adjusted so that on the one hand parts of the X-ray beam which are only slightly attenuated by the object are slightly attenuated by the X-ray filter and that, on the other hand, parts of the X-ray beam which are substantially attenuated by the object are transmitted by the X-ray filter substantially without attenuation. Because the brightness values of the X-ray image lie in a limited range, further processing of the X-ray image so as to achieve a clear reproduction of small details of low contrast, is very well possible.
The X-ray filter of the known X-ray examination apparatus comprises a very large number of glass capillary tubes, one end of each of which communicates with the X-ray absorbing liquid. The adhesion of the X-ray absorbing liquid to the inner wall of such a capillary tube is dependent on the electric voltage applied to the relevant capillary tube. Notably the contact angle enclosed by the X-ray absorbing liquid relative to the inner wall of such a capillary tube is dependent on the electric voltage difference between the inner wall and the X-ray absorbing liquid. When the contact angle is larger than 90.degree., the X-ray absorbing liquid hardly enters such a capillary tube; when the contact angle is less than 90.degree., such a capillary tube is filled with a quantity of X-ray absorbing liquid which is dependent on the magnitude of the electric potential difference. The quantity of X-ray absorbing liquid in each of the capillary tubes is controlled on the basis of the electric voltages applied to the capillary tubes.
The manufacture of the X-ray filter of the known X-ray examination apparatus is an intricate and hence expensive operation, because it is difficult to bundle a very large number of capillary tubes in a regular pattern.